Kayentachelys aprix has previously been hypothesized to be the oldest stem
cryptodiran turtle because of the presence of a flat, vertical plate on
the
processus pterygoideus externus, and the presence of a processus
trochlearis
oticum.

Just these two characters?

However, the presence of these characters cannot be confirmed in the
available specimens. Other putative stem-cryptodiran characters, such as
the
prefrontal-vomer contact and the presence of an epipterygoid, are herein
corroborated as being symplesiomorphies, because they generally appear to
be
present in basal turtles.